With reference to FIG. 2, a conventional amplifier 200 comprises a first transistor 210 and a second transistor 220, wherein a gate terminal 211 of the first transistor 210 receives a first bias voltage via a first bias resistor 230, a source terminal 212 of the first transistor 210 is grounded via a first inductor 240, a drain terminal 213 of the first transistor 210 receives a power source via a first loading resistor 250, a gate terminal 221 of the second transistor 220 receives a second bias voltage via a second bias resistor 260, a source terminal 222 of the second transistor 220 is grounded via a second inductor 270, and a drain terminal 223 of the second transistor 220 receives a power source via a second loading resistor 280. The amplifier 200 receives an input signal νs via the source terminal 212 of the first transistor 210 and the source terminal 222 of the second transistor 220 and outputs an amplifying signal νs via the drain terminal 213 of the first transistor 210 and the drain terminal 223 of the second transistor 220, wherein the noise figure of the amplifier 200 can be expressed as followed:
      F    =          1      +              γ        α            +                        4          ⁢                      R            S                                    R          L                      ,wherein γ is the channel heat noise coefficient, α is the ratio between the transconductance value of the first transistor 210 and the second transistor 220 and the conductivity of drain electrode under zero bias voltage, Rs is the input matching impedance, RL is the first loading resistor 250 and the second loading resistor 280. Therefore, following with the description of mentioned equation, under the condition that the amplifier 200 satisfies input matching (Rs, γ and α are fixed values), the amplifier 200 fails to lower the noise except raising the magnitude of the first loading transistor 250 and the second loading resistor 280.